Wind Turbine Monopile Foundations Research Articles

New shape function solutions for fracture mechanics analysis of offshore wind turbine monopile foundations

Offshore wind turbines are considered one of the most promising solutions to provide sustainable energy. The dominant majority of all installed offshore wind turbines are fixed to the seabed using monopile foundations. To predict the lifetime of these structures, reliable values for shape function and stress intensity factor are needed. In this study, a new equation is developed through finite element simulations which have been performed for a wide range of monopile geometries with different dimensions, crack lengths and depths, to evaluate shape function and stress intensity factor solutions for monopiles. The new solutions have been verified through comparison with the existing solutions provided by Newman & Raju for small hollow cylinders. The empirical shape function solutions developed in this study are employed in a case study and the results have been compared with the existing shape function solutions. It is found that the old solutions provide inaccurate estimations of fatigue crack growth in monopiles and they underestimate or overestimate the fatigue life depending on the shape function solution employed in the structural integrity assessment. The use of the new solution will result in more accurate monopile designs as well as life predictions of existing monopile structures.

Experimental and Numerical Studies on the Dynamic and Long-Term Behavior of Offshore Wind Turbines in Clay

Monopile foundations for offshore wind turbines (OWTs) are exposed to long-term dynamic loads from wind and waves. Hence, the long-term dynamic behavior of a monopile supported OWT is necessary for the sake of stability during the serviceable period. To assess the safety of the system, the serviceability limit state regarding the allowable tilt of the monopile at mudline is satisfied, and the fundamental frequency of the system is kept away from the rotor and blade passing frequencies. The present study investigates the long-term performance of monopile supported OWT in clay using a series of scaled model tests. The fundamental frequency and damping of an OWT system and the rotation and lateral deflection of a monopile head are examined for various load cycles with different amplitudes. The effect of long-term loading cycles and amplitude on the soil-pile stiffness is evaluated. It is observed that the fundamental frequency of the system decreases with the number of load cycles, whereas damping of the whole system is increased. The responses regarding accumulated rotation and deflection at monopile head are found to be increasing with the number of load cycles. For the numerical simulation, soil is modeled as viscoelastic material with a stiffness degradation function in three-dimensional finite element analysis. Finally, the fundamental frequency and response of the OWT obtained from the model test are validated with numerical results. A good agreement is observed between experimental and numerical results.

Monopiles subjected to uni- and multi-lateral cyclic loading

Offshore wind turbines are subjected to significant environmental loads from a combination of current, wind and wave action. Under such conditions, the directions of these environmental loads vary over the service life of the structure and therefore the cyclic lateral loading on the foundation also changes direction. The work reported in this paper examines the effects of multi-directional loading on the performance of offshore wind turbine monopile foundations. Tests were carried out in a model sand bed and a mobile loading platform was manufactured to apply loading on the pile in various directions. Tests were also carried out where the cyclic loading was applied under both one-way and two-way loading. The observations indicate significant differences in the stiffness of monopiles between uni-directional and multi-directional lateral cyclic loading. Multi-directional lateral cyclic loading generally results in higher displacements and lower stiffness compared to uni-directional loading, most likely due to shear deformation of a larger volume of soil mass adjacent to the pile.

Ship collision analysis on offshore wind turbine monopile foundations

An offshore wind farm can be located close to traffic lanes of commercial and passenger ships, which may lead to possible collision. The work presented in this paper aims to understand both the crushing behaviour and the nacelle dynamics of a monopile offshore wind turbine when impacted by a ship. Another objective was to deeply investigate the influence of various parameters like ship impact velocity and location, wind direction, soil stiffness and deformability of the striking ship.First, nonlinear numerical simulations of ship - Offshore Wind Turbine (OWT) collisions have been carried out with a rigid striking ship for a better understanding of the OWT's structural behaviour during collision. Different configurations for the wind turbine's structure have been used in order to highlight the modifications in behaviour induced by changing the soil conditions or the loading scenario. The resulting resistant force and internal energies have been compared as well as the tower top (nacelle) displacements and accelerations. Then, another series of simulations have been performed with a deformable ship in order to investigate the influence of the deformability of the striking ship on the OWT's behaviour.It was shown that a slight variation of the impact velocity can lead to consequences ranging from minor damage of the OWT to collapse. Furthermore, the behaviour of the OWT during collision is highly sensitive to wind loads as it was shown that in some cases the OWT can collapse for an impact velocity of only 3 m/s and, in the worst case, can fall directly on the ship. The results obtained showed that it is important to account for the soil flexibility when performing collision simulations because considering the monopile as being clamped at the base would lead to an overestimation of the plastic deformation of the collided structure. Moreover, when the OWT is collided by a deformable striking ship, the deformations of the OWT are 2 times smaller comparing to the deformations caused by impact with a rigid ship and the structure can withstand collisions without collapsing with an impact velocity up to 6 m/s.

P-y-ẏ curves for dynamic analysis of offshore wind turbine monopile foundations

The well-known p-y curve method provides soil-structure interaction that is independent of the load rate. In this paper an improved p-y curve method is proposed by considering the influence of the excitation frequency. For this purpose, a two-dimensional finite-element program is developed for analysis of a segment of an offshore monopile foundation placed in different depths. The intended use of the model is analyzes of offshore wind turbines in operation where small-magnitude cyclic response is observed in addition to the quasi-static response from the mean wind force. The response to small-magnitude cyclic loading is analyzed by employing coupled equations based on the u-P formulation, i.e. accounting for soil deformation as well as pore pressure. Thus, the paper has focus on the effects of drained versus undrained behavior of the soil and the impact of this behavior on the stiffness and damping related to soil-structure interaction at different load frequencies. In order to enable a parameter study with variation of the soil properties, the constitutive model is purposely kept simple. Hence, a linear poroelastic material model with few material parameters is utilized. Based on the two-dimensional model, linear p-y-ẏ curves are extracted for the lateral loading of monopiles subjected to cyclic loads. The developed code is verified with findings in the literature.

Natural frequency degradation and permanent accumulated rotation for offshore wind turbine monopiles in clay

Offshore wind turbine (OWT) monopile foundations are subjected to cyclic loading from wind, waves, and operational loads from rotating blades. Lateral monopile capacity can be significantly affected by cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency degradation and accumulated permanent rotation at the mudline, respectively. This paper introduced novel combinations of existing p-y curve design methods and compared the impact of short-term cyclic loading on monopiles in soft, medium, and stiff clay. The results of this paper indicate that short-term cyclic loading from extreme storm conditions are unlikely to significantly affect natural frequency and permanent accumulated rotation for OWT monopiles in stiff clays, but monopiles in soft clay may experience significant degradation. Further consideration is required for medium clays, as load magnitude played a strong role in both natural frequency and permanent rotation estimation.

Recovery-based error estimation in the dynamic analysis of offshore wind turbine monopile foundations

Offshore wind turbine foundations are affected by cyclic loads due to oscillatory kinematic loads, such as those from wind, waves, and earthquakes. Monopiles are often used as a foundation concept for offshore windmill turbines. In this study, coupled dynamic equations with the u-P formulation for low-frequency load are considered for an offshore wind turbine monopile foundation, to present the response in terms of pore water pressure (PWP), stress and strain distribution in an elastic porous medium at regions around the monopile foundation. Different stress recovery techniques based on the Zienkeiwicz–Zhu (ZZ) error estimator namely, super-convergent patch recovery (SPR), weighted super-convergent patch recovery (WSPR), and L2-projection techniques are also investigated to recover the stresses at nodal points in the finite element method. To estimate errors in the time domain when performing transient simulations, three recovery processes are used with different meshes. The convergence of the dynamic problem is also studied. The results are verified with findings in the literature, revealing that the time period of effective stresses follows the applied load frequency. In conclusion, the history of the shear stress can have an important effect on the shear stress distribution, making it asymmetric in the time domain.

Behavior of cyclically loaded monopile foundations for offshore wind turbines in heterogeneous sands

This paper presents a probabilistic assessment of the long term performance of monopile foundations for offshore wind turbines in dense sand with spatially varying stiffness. Response of the monopile foundation to long term cyclic loading is simulated by coupling a three-dimensional finite element pile–soil model with a stiffness degradation material model. With the implementation of random field model of sand stiffness variability, this study provides a discussion on the probabilistic effects in the long term response of the pile–soil system. The probabilistic response is evaluated with respect to the serviceability and ultimate limit states of monopile foundations in the Monte Carlo framework for a set of monopile embedment lengths and lateral loading cycles. The statistics associated with the monopile displacements, rotations and bending moments demonstrate the influence of the monopile embedment length and the number of lateral loading cycles on the long term probabilistic response of monopile foundations. The estimated probabilities of exceeding the limit states revealed the serviceability limit state as being dominant in this study and indicated the importance of the installation tolerance on the long term response of monopile foundations.

Sensitivity of Wave Fatigue Loads on Offshore Wind Turbines under Varying Site Conditions

Considerable variations in environmental site conditions can exist within large offshore wind farms leading to divergent fatigue loads on support structures. An efficient frequency-domain method to calculate wave-induced fatigue loads on offshore wind turbine monopile foundations was developed. A verification study with time-domain simulations for a 4MW offshore wind turbine showed result accuracies of more than 90% for equivalent bending moments at mudline and interface level. This accuracy and computation times in the order of seconds make the frequency-domain method ideal for preliminary design and support structure optimization. The model is applied in sensitivity analysis of fatigue loads using Monte-Carlo simulations. Local and global sensitivity studies and a probabilistic assessment give insight into the importance of site parameters like water depth, soil stiffness, wave height, and wave period on fatigue loads. Results show a significant influence of water depth and wave period. This provides a basis for design optimization, load interpolation and uncertainty analysis in large wind farms.

Influence of second-order random wave kinematics on the design loads of offshore wind turbine support structures

The impact of wave model nonlinearities on the design loads of wind turbine monopile foundations is delineated based on a second-order nonlinear random wave model that satisfies the boundary conditions at the free surface and by including the effects of convective acceleration in the inertial loads. The second-order nonlinear water kinematics is developed based on a Gram Charlier series expansion using the first four stochastic moments of the wave process. The wave surface velocities and accelerations are expressed using a Taylor series expansion about the mean sea level, which satisfies to the second-order, the unsteady Bernoulli equation and normal flow condition at the free surface. The operating design loads on the monopile are computed using fully coupled and uncoupled methods. The computation of the mud level loads based on the inclusion of nonlinear wave surface kinematics is compared with those obtained when using linear waves and Wheeler stretching. The effect of the spatial derivatives of the wave velocity on the wave surface kinematics is quantified and shown to determine the wave spectral cut-off frequency limit. The spatial derivatives of wave velocity also participate in the expression for the wave convective acceleration, whose effect is demonstrated on the inertial loads on the foundation in the presence of ocean currents. The effect of nonlinear water kinematics on the monopile design load reveals the large frequency bandwidth of wave structure interaction, but the phase differences between the hydrodynamic loads with the rotor loads tend to lower the probability of joint simultaneous extreme peaks in hydrodynamics and rotor loads.

Soil–structure reliability of offshore wind turbine monopile foundations

AbstractAn overview of offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. The uncertainty in offshore soil conditions as well as random wind and wave loading is currently treated with a deterministic design procedure, though some standards allow engineers to use a probability‐based approach. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute p‐y method, which is problematic for large OWT pile diameters. Probabilistic methods are used to examine the reliability of OWT pile foundations under serviceability limit states using Euler–Bernoulli beam elements in a two‐dimensional pile–spring model, non‐linear with respect to the soil springs. The effects of soil property variation, pile design parameters, loading and large diameters on OWT pile reliability are presented. Copyright © 2014 John Wiley & Sons, Ltd.

Monopile foundations for offshore wind turbines – solutions for greater water depths

AbstractA study into the feasibility of monopile foundations at sites in the North Sea with water depths as great as 35 m has been carried out in which various options for the geometric configuration and the selection of material were considered. In parameter studies, simplified design methods were applied to assess the effects of the individual load components at the draft design stage. The fatigue limit state becomes more and more relevant as the water depth increases; therefore, dynamic effects must be examined with special care. Turbine concepts with low RNA mass and low rated speed help to achieve the desired design in the soft‐stiff regime. As a result, it can be said that monopile foundations with their great manufacturing advantages can be constructed for water depths beyond the current limits of practical experience if the logistical challenges in handling large masses are solved.

SIMULATION OF EXTREME WAVE INTERACTION WITH MONOPILE MOUNTS FOR OFFSHORE WIND TURBINES

Extreme wave run-up and impacts on monopile foundations may cause unexpected damage to offshore wind farm facilities and platforms. To assess the forces due to wave run-up, the distribution of run-up around the pile and the maximum wave run-up height need to be known. This paper describes a numerical model AMAZON-3D study of wave run-up and wave forces on offshore wind turbine monopile foundations, including both regular and irregular waves. Numerical results of wave force for regular waves are in good agreement with experimental measurement and theoretical results, while the maximum run-up height are little higher than predicted by linear theory and some empirical formula. Some results for irregular wave simulation are also presented.

Pile driving zone of responsiveness extends beyond 20 km for harbor porpoises (Phocoena phocoena (L.))

Behavioral reactions of harbor porpoises (Phocoena phocoena) to underwater noise from pile driving were studied. Steel monopile foundations (4 m diameter) for offshore wind turbines were driven into hard sand in shallow water at Horns Reef, the North Sea. The impulsive sounds generated had high sound pressures [source level 235 dB re 1 microPa(pp) at 1 m, transmission loss 18 log(distance)] with a strong low frequency emphasis but with significant energy up to 100 kHz. Reactions of porpoises were studied by passive acoustic loggers (T-PODs). Intervals between echolocation events (encounters) were analyzed, and a significant increase was found from average 5.9 h between encounters in the construction period as a whole to on average 7.5 h between first and second encounters after pile driving. The size of the zone of responsiveness could not be inferred as no grading in response was observed with distance from the pile driving site but must have exceeded 21 km (distance to most distant T-POD station).

Concrete is the Future for Offshore Foundations

This paper compares the costs of using concrete foundations against steel monopile foundations for offshore wind turbines, and argues that concrete foundations will be cheaper. Most offshore windfarms have steel monopile foundations, but in Denmark concrete gravity foundations have been used with success. Two projects have tendered for steel monopiles and for concrete foundations and have implemented the concrete foundations that proved cheaper. No project has tendered for both foundation concepts and chosen steel monopiles. Nysted Offshore Windfarm with concrete foundations has the cheapest foundations of any offshore windfarm so far. A conceptual foundation study carried out for the London Array West Offshore Windfarm indicates that the same method and very low-cost foundations as for Nysted can be used. Optimised design of light-weight concrete constructions is the key to low-cost installation. Cheap manufacture can be carried out near the site or at even lower cost in Eastern Europa from where it can be shipped at little extra cost. The main construction of steel monopile foundations will become twice as costly as of concrete gravity foundations, and though installation is more costly for the gravity foundations it seems most likely that tendering between steel monopile and concrete gravity for London Array West will prove concrete considerably cheaper. It is argued that these considerations are to a wide extent generally valid, and also for very large turbines in deeper water. Concrete foundations will in 2006 be installed at Lillegrunden Offshore Windfarm in Sweden, and at Belgian Thornton Bank in 2006–7. So indications are strong that concrete is the future for offshore foundations.